Process for the recovery of nitric oxide from gaseous mixtures



Patented June 30, 1942 PROCESS FOR THE RECOVERY OF NITRIC OXIDE FROM GASEOUS MIXTURES Kenneth H. Hoover, Terre Haute, Ind., assignor to Commercial Solvents Corporation, Terre Haute, Ind; a corporation of Maryland No Drawing. Application August 2, 1940,

- Serial No. 350,150

' Claims.

My invention relates to the recovery of nitric oxide and hydrocarbons from gaseous mixtures, and more specifically to the recovery of nitric oxide and parafiin hydrocarbons from the gaseous reaction products resulting from the vapor phase nitration of parafiin hydrocarbons.

The paraffin hydrocarbons and especially those of low molecular weight, may be successfully nitrated in the vapor phase, for example, in accordance with the procedures of U. S. Pats. 1,967,667 and 2,071,122 of H. B. Hass et al., 2,161,475 and 2,164,774 of G. K. Landon, and 2,206,813 of H. B. Hass and E. B. Hodge. In accordance with these procedures, the hydrocarbon is mixed with nitric acid or nitrogen dioxide, preferably having a considerable molar excess of hydrocarbon, and the resulting mixture is passed through a heated reaction chamber. The reaction products include, as well as nitroparamns, unreacted paraffin hydrocarbons and carbon ox-. ides. In all cases there is a substantial proportion of nitric oxide in the gaseous reaction prodnets, and since nitric acid or nitrogen dioxide are the most expensive raw materials used in the process, it is evident that recovery of the nitric oxide, for conversion to nitrogen dioxide or nitric acid, is economically desirable.

It has previously been proposed to recover nitric oxide from the gaseous reaction products by scrubbing with an acid solution of ferrous sulfate, and subsequently liberating nitric oxide from the scrub liquor by the use of high temperatures or low pressures, or both. This process, however, entails serious corrosion difiiculties, and the expense of handling large volumes of scrub liquor at high temperatures or low pressures, makes this procedure commercially undesirable.

Since the purpose of recovering nitric oxide from the reaction products is to re-oxidize it to nitrogen dioxide for re-use in the process, there also existed the possibility of oxidizing the nitric oxide in the presence of some of the remaining constituents of the gaseous mixture. For example, after removal of the hydrocarbons, which constitute the bulk of the gas mixture, the remaining mixture of nitric oxide and carbon oxides might possibly be directly subjected to oxidation. However, when sufficient air or oxygen is added to such mixtures to effect the oxidation of the nitric oxide, inflammable or explosive mixtures are produced. This obviously precludes the use of such a process. Y

In co-pending application Serial No. 328,527, a process is described in which the nitric oxide is recovered by oxidation in the presence of a suflicient amount of the hydrocarbon being nitrated to avoid the formation of inflammable mixtures. This process is especially applicable when nitrating low molecular weight hydrocarbons. When nitrating a relatively high molecular weight hydrocarbon, however, it may be necesary, or desirable, to remove the unreacted hydrocarbon from the nitration reaction products before oxidizing the nitric oxide, because of the fact that under the conditions under which oxidation of nitric oxide occurs, said unreacted hydrocarbon is in liquid form. As a result, the hydrocarbon content of the gaseous reaction mixture is reduced to such an extent that explosive mixtures are produced. In accordance with my present invention, the -hydrocarbon thus re-' moved is replaced by a gaseous hydrocarbon, or by any gas which does not react with the other constituents of the mixture, in an amount sufficient to prevent the formation of inflammable mixtures when air is subsequently introduced into the mixture. In accordance with my process, therefore, the unreacted hydrocarbon in the nitration reaction products is removed, either in part or substantially completely; a gaseous hydrocarbon or unreactive gas is introduced in sufficient amounts so that inflammable mixtures are not formed on the addition of air; then air is introduced, and the oxidation of the nitric oxide is carried out in the usual manner.

After removal of the nitrogen dioxide formed by the oxidation of the nitric oxide, the treatment of the remaining gases will vary, depending upon their composition. Thus, if a relatively large part of the gas is unreacted hydrocarbon irom the nitration mixture, it will be advantageous to recover this, as, for example, by scrubbing with oil in the known manner. If most of the remaining gas consists of the added gas, this may be recycled without further treatment, or may be vented to the atmosphere.

The amount of gas added prior to the oxidation of ihe nitric oxide will depend upon the nature of the added gas and upon the composition of the mixture after the addition of air or oxygen. If the added gas is a hydrocarbon, the amount necessary may be judged as described in co-pending application Serial No. 328,527 using the chart constituting Figure II of the paper Limits of Flammability of Mixtures of Propane, Air,and Nitrogen Dioxide (Ind. and Eng. Chem. 30, 1390). In using this chart, the total weight of hydrocarbon is used instead of the weight of propane. Thus, it can be seen that the hydrocarbon should make up at least 40 and preferably 60% by weight of the gaseous mixture after the addition of air or oxygen. When employing other diluent gases such as nitrogen, or carbon dioxide, the amount again will depend upon the other constituents of the mixture, and maybe determined experimentally according to known methods as described in Bulletin No. 279 issued by the U. S. Bureau of Mines (1928, revised in 1931). In general, I prefer to use at least a 50% excess of the added diluent gas over that necessary to prevent the formation of inflammable mixtures, or over that required to slow the rate of vertical free flame propagation to not over five feet per second.

My process is particularly applicable to prod-.

ucts of the nitration of hydrocarbons containing four or more carbon atoms. However, it may be desirably applied at times to products from the nitration of hydrocarbons with three or less carbon atoms. It is particularly advantageous in the former case, however, since it permits the oxidation of the nitric oxide at relatively high pressures-and low temperatures (both of which conditions accelerate the oxidation of nitric oxide), without condensation of hydrocarbon taking place.

The pressure and temperature at which the oxidation of nitric oxide may be carried out may be judged from the concentration of materials present in the mixture and known data concerning the dew points of the constituents of the mixture.

The gaseous reaction products from the nitra tion reaction, remaining after recovery of the bulk of the nitroparaffins by condensation, are preferably, but not necessarily, subjected to water scrubbing prior to eifecting the oxidation of the nitric oxide. This water scrubbing, which may suitably be carried out in accordance with the procedure of U. S. Pat. 2,150,123 of J. Martin and E, B. Hodge, serves to remove aldehydes, ketones, residual nitro-parafiins and other water-soluble materials from the gaseous mixture.

The oxidation reaction may be effected in accordance with prior practices in this regard, for example, in accordance with the procedure commonly used in ammonia oxidation plants for the production of nitric acid. The reaction is effected simply by mixing air or oxygen with the gaseous mixture containing the nitric oxide, and passing the resulting mixture through a reaction chamber at a rate suiliciently slow to obtain substantially complete reaction. Low temperatures and high pressures increase the velocity of the reaction, and for this reason a combination of both is preferable. Refrigeration is usually not warranted from a cost standpoint, and prevailing atmospheric temperatures or cooling-Water temperatures may be satisfactorily employed. Increase in pressure permits greatly increased space velocity, but the expense of compression of the gases above the pressure employed for the nitration reaction is usually not warranted. Any pressure which may be employed for the nitration reaction itself will be satisfactory for the oxidation reaction ii the space velocity is ad- Justed in accordance with known practices to insure complete reaction at the pressure employed.

Oxygen or an oxygen-containing gaseous mixture free from constituents which would interfere with the reaction or the subsequent recovoxygen theoretically required for the oxidation is shown by the following equation:

However, if the resulting nitrogen dioxide is to be recovered by absorption in water to form nitric acid, the amount of oxygen theoretically required for the reactions involved, is represented by the following equations:

which may be expressed by the single equation:

4NO+3O2+2H2O*4HNO3 Any amount of air, from the theoretical requirement to a very considerable excess over this, may be used. Unduly excessive amounts of air entail the expense of handling unnecessarily large volumes of gases, and too large an excess of air will, of course. reduce the hydrocarbon content of the mixture sufiiciently to produce a flammable mixture. I prefer, therefore, to use amounts of air ranging from the theoretical amount to 100% in excess of this, and preferably an amount representing 10%50% in excess of the theoretical amount.

At the conclusion of the oxidation reaction the resulting nitrogen dioxide may be recovered in any suitable manner, as, for example, by fractional condensation of the gas mixture. In such a process precautions should be taken to prevent the formation of mixtures of liquid nitrogen dioxide and liquid hydrocarbons, as such mixtures may be very powerful explosives,

Since it is usually desirable to recycle the nitrogen dioxide in the process, it is suitably recovered in the form in which it is used in the nitration reaction. Thus, the nitrogen dioxide may be absorbed in water to form nitric acid, if nitric acid constitutes the nitrating agent used in the nitration reaction. This absorption step may conveniently be used to recover the nitrogen dioxide from the gas mixture leaving the oxidizery operations, may be used as the oxidizing ing chamber. For this purpose the usual type of absorption tower may be used, and prior practices may be followed in all respects. Here, again, low temperature is desirable in order to obtain nitric acid of high concentration, but refrigeration is usually unwarranted from a cost standpoint. Available cooling-water temperatures will usually be satisfactory, and nitric acid of satisfactory strength for recycling in the process can be obtained at usual prevailing at: mospheric temperatures. After absorption of the bulk of the nitrogen dioxide by water scrubbing, the remaining traces may be removed from the gas mixture by scrubbing with an alkaline medium, as, for example, aqueous caustic soda, or soda ash. This is particularly desirable if the hydrocarbon is to be recovered from the residual gases, since even small amounts of nitrogen dioxide may cause serious corrosion in subsequent recovery apparatus, due to the presence of water vapor in the gases leaving the water scrubber, resulting in nitric acid formation in the residual gas mixture. Various modified methods for recovering the nitrogen dioxide from the gas mixture will of course be evident to those skilled in the art, and my invention is not limited to any particular method in this regard.

After removal of the nitrogen dioxide from the gaseous mixture, the remaining gas will consist of excess oxygen, nitrogenif air was used,

as the oxidizing medium, oxides of carbon, unreacted hydrocarbon from the nitration mixture,

cycled, or it may be discarded and fresh gas introduced continuously.

My invention may be further illustrated by the following specific example:

Example The reaction products from the vapor phase nitration of, butane by means of nitric acid were cooled to approximately 20 C. to condense out the nitroparafiins and water and the bulk of the butane. The remaining gas mixture was scrubbed with water and then had approximately the following analysis (per cent by volume):

Butane 22.5 Nitric oxide 52.5 Unsaturates 12.5 Carbon monoxide 7.5 Carbon dioxide 5.0'

To this was added methane to form a mixture of the following composition (per cent by volume) Methane 83.4

Butane 3.7 Nitric oxide 8.7 Unsaturates 2.1 Carbon monoxide 1.2

Carbon dioxide 0.9

This gas mixture, at a temperature of about C., was mixed with air at the same temperature at a rate of approximately 136 cubic feet of air per hour for 360 cubic feet of gas mixture per hour. The resulting mixture was passed through an oxidation chamber at a rate of approximately 800 cu. ft. per hour per cubic foot of reaction space. The exit gas mixture was found to have a temperature of approximately 60 C., due to the exothermic nature of the reaction. This gas was cooled to approximately 20 C., and passed through a 33-plate bubble-cap column countercurrent to water entering at a temperature of approximately 20 C. The rate of water flow was controlled so as to obtain, as the solution leaving the column, nitric acid of approximately 1.30 specific gravity. In a continuous process involving the initial rates of gas flow specified above, water introduced into the column at approximately one gallon per hour was found to produce nitric acid of approximately 1.30 specific gravity. The gas mixture leaving the column was vented to the atmosphere.

In the above example the gases were maintained at a pressure of 80 lbs. per sq. in. (gauge) throughout the entire process, but volumes specified are based on atmospheric pressure and 26 C.

It is to be understood, of course, that the above example is merely illustrative and does not limit the scope of my invention. Gaseous reaction products from the nitration of other parafiin hydrocarbons, and using other nitration conditions, may be satisfactorily treated by my process, if sufficient diluent gas is provided to prevent the formation of flammable mixtures, and if the constituents of the resulting gaseous mixture are maintained in the gaseous state during the oxidation and during the recovery of the resulting nitrogen dioxide.

uent gases may be substituted for the methane employed in the example. By the term diluent gas, as used here and in the appended claims, is meant a gas which is inert to the other constituents of the gaseous mixture, under the reaction conditions of the oxidation and recovery steps of the process, and which liquefles at a temperature lower, than the temperature of liquefaction of the unreacted hydrocarbon removed from the reaction products. The conditions for the oxidation of the nitric oxide and for the recovery of the resulting nitrogen dioxide, specified in the above example, may also be varied in numerous respects, in accordance with prior practices. 'My process may also be used in conjunction with other gas recovery or purification steps. In general, it may be said that the use of any equivalents or modifications of procedure, which would naturally occur to one skilled in the art, is included in' the scope of my invention. 1 4

My invention now having been described. what I claim is:

1. A process for recovering nitric oxide from gaseous reaction products of the vapor phase nitration of a paraflin hydrocarbon containing more than three carbon atoms, said gaseous reaction products containing unreacted hydrocarbon and nitric oxide, which comprises removing at least a part of the unreacted hydrocarbon from said gaseous mixture, incorporating in said gaseous mixture sufficient of a hydrocarbon containing less than three carbon atoms, so that the mixture, effecting reaction between said oxygenv and said nitric oxide, at a temperature sufiicient- 1y high and a pressure sufficiently low to maintain said hydrocarbons in the gaseous phase, and removing nitrogen dioxide from the resulting gase ous mixture, at a temperature sufficiently high and a pressure sufficiently low to maintain said hydrocarbons in the gaseous phase.

2. A process for recovering nitric oxide from gaseous reaction products of the vapor phase nitration of a hydrocarbon having more than three carbon atoms, said gaseous reaction products containing unreacted hydrocarbon and nitric oxide, which comprises removing at least a part of the unreacted hydrocarbon from said gaseous mixture, incorporating in said gaseous mixture sufiicient methane so that the hydrocarbon content of the resulting gaseous mixture, after incorporating air therein in an amount ranging from that theoretically required to oxi dize said nitric oxide, to 100% in excess of such amount, constitutes at least 40% by weight of hydrocarbons, then incorporating said amount of air, effecting reaction between said nitric oxide and the oxygen of said air, at a temperature sufficiently high and a pressure sufliciently low to maintain the hydrocarbons in the gaseous phase, and recovering nitrogen dioxide from the resulting gaseous mixture, at a temperature sufficient- 1y high and a pressure sufliciently low to main- Likewise, various diltain the hydrocarbons in the gaseous phase.

3. A process for recovering nitric oxide from gaseous reaction products of the vapor phase reacted butane, incorporating in said gaseous 1 ranging from that theoretically required to oxi- 5 dize said nitric oxide, to 100% in excess of such amount, constitutes at least 40% by weight of hydrocarbons, then incorporating said amount of air, effecting reaction between said nitric oxide and the oxygen 01' said air, at a temperal0 ture sufiiciently high and a pressure sufllciently low to maintain the hydrocarbons in the gaseous phase, and recovering nitrogen dioxide from the resulting gaseous mixture at a temperature sufficiently high and a pressure sufliciently low to maintain the hydrocarbons in the gaseous phase. 4. In a process for recovering nitric oxide by oxidation of the nitric oxide and recovery oi the resulting nitrogen dioxide from gaseous reaction products of the vapor phase nitration of a paraifin hydrocarbon, said products containing unreacted hydrocarbon and nitric oxide, wherein the hydrocarbon liquefies under the conditions employed for so oxidizing the nitric oxide, thereby tending to form an explosive gaseous mixture hydrocarbon and replacing thesame with a 80 diluent gas that is normally gaseous at the reaction temperature in such amounts that the formation of an explosive mixture is avoided on subsequent addition or sufllcient oxygen to oxinitric oxide, but insufilcient to form a flammable mixture, effecting reaction between said oxygen and said nitric oxide, recovering nitrogen dioxide from the resulting gaseous mixture and maintaining the other constituents of the gaseous mixture in the gaseous phase throughout 'said oxidation and recovery steps.

5. In a process for recovering nitric oxide by oxidation of the nitric oxide and recovery of the resulting nitrogen dioxide from gaseous reaction products of the vapor phase nitration of a para!- fin hydrocarbon, said products containing unreacted hydrocarbon and nitric oxide, wherein the hydrocarbon liquefies under the conditions employed for so oxidizing the nitric oxide, thereby tending to form an explosive gaseous mixture due to a decrease in the gaseous hydrocarbon content of the mixture, the improvement which comprises avoiding the formation of the explosive mixture by the removal of at least a portion of the liquefiable hydrocarbon and replacing the same with a diluent gas that is normally gaseous at the reaction temperature in such amounts that the formation of an explosive mixture is avoided on subsequent addition of sufflcient oxygen to oxidize said nitric oxide to nitrogen dioxide, adding sufiicient oxygen in said mixture to oxidize said nitric oxide, but insufilcient to form a flammable mixture, effecting reaction between said oxygen and said nitric oxide, recovering nitrogen dioxide from the resulting gaseous mixture and maintaining the other constituents of the gaseous mixture in the gaseous phase throughout said oxidation and recovery .dize said nitric oxide to nitrogen dioxide, adding steps. sumcient oxygen in said mixture to oxidize said KENNETH H. HOOVER. 

